The process of forming a coating in a mold was developed to reduce the cost of producing an attractive surface finish on reaction-injection-molded (RIM) parts. The process uses compatible urethane chemistry for the coating and substrate to form the decorative and protective surface layer “coating” simultaneously with the structural substrate layer. In-mold coatings exhibit cross-linked bonding to the RIM substrate and they can be custom formulated to match physical properties with a wide range of polyurethane substrates, including structural foams and elastomers. The in-mold coating process with its unique approach to integrating flexible coatings with rugged polyurethane substrates is a natural fit for manufacturing components used is market segments such as heavy trucks, agriculture and construction, boating, and lawn-and-garden. FIG. 1 shows a sectional view of a prior-art in-mold coated surface, in which the pigmented coating 10 is covalently bonded to the polyurethane substrate 14. (As the coating 10 preferably is between about 1.5 and 2 thousandths of an inch in thickness (dry film thickness) whereas the substrate 14 can be up to an inch or more in thickness, FIG. 1 is of course not drawn to scale.)
As a result of the conventional process of in-mold coating, the substrate is covered by a layer of pigmented coating, as shown in FIG. 1. In this process, the components that form the pigmented coating 10 are first sprayed onto the mold surface, while the mold is open, so as to form a thin layer. Then the material that forms the substrate is applied onto the pigmented layer. Typically, the substrate-forming material is applied by injecting it into the mold while the mold is closed. In order to be able to properly mix and spray the coating components to create a coating with a satisfactory finish, solvents are included by the coating formulator. The solvents provide a sprayable viscosity to the coating components, so that a uniform thin layer can be sprayed onto the mold. In contrast, the substrate-forming material should be solventless. In-mold coating can yield numerous benefits including a high gloss “Class A” finish without “orange peel,” runs, or dirt. Significantly, such attributes can generally be achieved with tremendous savings of time and material compared to conventional spray painting methods. Physical property and chemical resistance tests have shown that in-mold coated surfaces perform in some respects as well as or better than post-mold painted surfaces. The process of in-mold coating eliminates the need for post-mold painting steps such as cleaning, degreasing, sanding, and priming. Those procedures can constitute up to half of a RIM product's total cost.
In-mold coatings differ substantially from conventional coatings that are applied after molding in which a layer of paint is sprayed on after the substrate is cured. Coatings applied after curing do not have the same opportunity to chemically bond to the substrate compared to coatings applied in the mold. Because in-mold coatings are formed simultaneously with the substrate, they are a chemically bonded, integral part of the composite. Consequently, in-mold coatings do not crack or peel as readily as post-mold painted coatings. Furthermore, a phenomenon termed “outgassing” occurs with RIM parts where gaseous processing and reaction byproducts escape from the freshly molded part. Outgassing can interfere with the drying/curing of coatings applied after molding, causing voids that ruin the finish. Steps to avoid outgassing interference such as waiting (storage) or post curing to accelerate the de-gassing add significant cost to the finishing process. Coatings produced with the in-mold coating process are unaffected by RIM substrate outgassing; the high quality finish is formed simultaneously with the substrate essentially before outgassing occurs.
Even though the pigments and resins may be the same in coatings formed by the in-mold process and coatings formed by the conventional post-mold spray application process, it has been reported that in-mold coatings sometimes fade more readily when exposed to direct sunlight. It is speculated in these cases that the reason for the difference lies in the physics of the application method used. For post-mold spray applied coatings, pigments can settle as the paint dries over a period of time, creating a stratified layer that is resin rich near the surface and pigment rich at a depth removed from the surface. The resin rich region near the surface can absorb UV radiation, therefore protecting the underlying pigments. By comparison, coatings formed by the in-mold coating process are more homogeneous, containing pigments more uniformly distributed throughout the coating layer because the liquid coating materials gel very quickly as a result of the hot mold.